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<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        function sum(x,y) {
            return lazysun = function (x, y) {
                return x + y;
            };
        }

        function add(){
            return 1;
        }

        add()//return的值了   1
        sum()
        /* return的函数了
          arr.reduce(function (x, y) {
                return x + y;
            }) */

        let num = sum();
        let num1 = sum([1, 2, 3, 4, 5]); // 15

        console.log(num(1,2));
    </script>
</body>
</html>